Distributed Temperature Sensing (DTS) sensors using optical fibers have been known for more than 20 years. The technology has evolved over the years and moved from the laboratory environment into the field in numerous applications, e.g. down hole sensing in oil & gas well, pipeline monitoring, or hot spot detection in industrial applications. The sensing probes are made out of telecommunication grade optical fibers cabled and deployed in the various applications.
Optical fibers used in communication systems are either single mode or multi mode. All optical fibers have a core and a cladding, and the core is normally 6-9 μm in diameter for single mode fibers or 50 μm and higher for multimode fibers while the diameter of the cladding is around 125 μm. Single mode fibers are normally step index fibers, i.e. the refractive index in the fiber core is different from the refractive index in the cladding to satisfy the light guiding conditions in Snell's law. The core of the single mode fiber can as the name implies only guide a single mode of the light traveling in the fiber. This will minimize dispersion and maintain a high bandwidth in the fiber.
Multi mode fibers can as the name implies carry multiple modes of light in the fiber. Multiple modes in a step index fiber causes signal dispersion as the different modes in the fiber can travel in many different paths and thereby reaching the receiver at different time. The way to mitigate signal dispersion in a multimode fiber is to introduce a graded index profile, which forces the various modes to travel with basically the same effective speed in the fiber. The better the graded index profile is optimized, the higher the bandwidth is in the graded index fiber.
The host material in optical fibers is fused silica, i.e. both the core and the cladding is mainly fused silica. The variations in refractive index are achieved by introducing various chemical, or dopants, in different concentrations during the fiber manufacturing process. The dopants and manufacturing methods are optimized for telecommunication type applications.
Optical fibers have been known to degrade rapidly when deployed in harsh environments like oil & gas wells where the temperature and pressure may be significantly higher than most telecommunication applications. The down-hole environment may also have a number of different chemicals that may react with the dopants in optical fibers. Hydrogen in particular, has been known to create severe attenuation in optical fibers with germanium doped core regions via a phenomena called hydrogen darkening.
Pure silica core fibers provide benefits in application of Distributed Temperature Sensing (DTS) for downhole environments, which have high temperatures and pressures and also contain hydrogen gases. Pure silica core has less susceptibility to the attenuations related to hydrogen darkening and lower transmission loss than conventional impurity-doped fibers. But the single mode version has issues of signal to noise ratio due to small light coupling and low Stimulated Raman Scattering threshold level due to its small core size.
The multi mode version could be a better solution. Multimode fibers typically have higher numerical apertures than single mode fibers. Higher numerical aperture means greater acceptance angles for input light into the fiber. Thus, fiber-to-fiber splices exhibit lower loss, fiber-to-device coupling is more efficient, and fiber-bending losses are lower. On the negative side, multimode fiber systems have an issue of higher inter modal dispersion (IMD), which broadens the input light signal. When an optical pulse is launched into a fiber, the energy in the pulse is distributed over all the propagating modes of the fiber. Each of the propagating modes travels at a slightly different speed along the fiber. As a result, the launched pulse is broadened significantly along the length of fiber. In distributed temperature sensing systems, this affects one of the critical parameters mentioned before—the spatial resolution, which is determined by the width of input pulse. When the pulse spreads more along distance, the spatial resolution determined by the pulse width is degraded more.
For downhole applications then, or any application in which reaction with typical dopants can create severe attention issues there is an important need for an optical fiber system that provides effective numerical aperture, temperature resolution, and spatial resolution in the presence of a high temperature/pressure hydrogen environment.